1. Technical Field
Example embodiments of the present invention relate in general to a wireless communication system and method of performing communication in the same, and more specifically, to a wireless communication system for improving error performance which deteriorates when network coding is employed in a relay-based cooperative wireless communication system environment, particularly, in an asynchronous transmission environment in which an uplink transmission rate and downlink transmission rate are different from each other, and a method of performing communication in the wireless communication system.
2. Related Art
Most general wireless communication environments are asynchronous transmission environments in which the amount of data of downlink signals transmitted by a base station to a terminal is larger than the amount of data of uplink signals transmitted by the terminal to the base station.
FIG. 1 is a conceptual diagram of an example of band use of a base station and a terminal in an asynchronous transmission environment.
As shown in FIG. 1, a downlink signal is transmitted from the base station to the terminal using the entire available band, and an uplink signal is transmitted from the terminal to the base station using half the available band.
Since the uplink signal uses half the entire available band, the uplink and downlink signals overlap each other in the one half of the available band but do not overlap in the other half of the available band.
Meanwhile, in a relay-based cooperative communication system, respective transmission steps are divided according to time sections to avoid interference caused by simultaneous transmission between the uplink and downlink. In other words, transmission from a base station to a relay, transmission from the relay to a terminal, transmission from the terminal to the relay, and transmission from the relay to the base station are performed in different time sections, and four time sections are required to exchange a signal between the base station and the terminal once.
FIG. 2 is a conceptual diagram of signal exchange between a base station and a terminal via a relay by such general time section division. Transmission is performed from the base station to the relay in time section 1, from the relay to the terminal in time section 2, from the terminal to the relay in time section 3, and from the relay to the base station in time section 4.
However, this method has a problem in that transmission efficiency decreases as the number of time sections increases. Thus, network coding schemes in a relay have been researched to reduce the number of time sections. In other words, such time section division reduces the overall transmission rate of a system. To solve this problem, a network coding scheme in which a part of time sections are combined, and interference resulting from overlap of time sections is removed by signal processing has been suggested.
Among network coding schemes, a complex field network coding (CFNC) scheme is attracting attention as a low-complexity network coding scheme which uses addition and subtraction of complex signals while reducing the required number of time sections to half that of a general transmission method.
FIG. 3 is a conceptual diagram of time section division when the CFNC scheme is employed. In time section 1, a base station and a terminal simultaneously transmit signals to a relay, and in time section 2, relay transmission signals to which the signals transmitted by the base station and the terminal are added are transmitted to the base station and the terminal. In this way, in a relay-based cooperative wireless communication environment, respective transmission steps are divided according to time sections to avoid interference caused by simultaneous transmission between an uplink and downlink.
However, this method has a problem in that error performance significantly deteriorates during a maximum likelihood (ML) determination process performed by a relay to separate overlapping uplink and downlink signals.